Such a leak detecting device is known from WO 96/24828. In a leak detecting device of this type, the sensitivity and the detection limit of the leak detecting device can be respectively adjusted. To this end, the high vacuum pump the inlet of which is connected with a mass spectrometer and the outlet of which is connected with a prevacuum pump is provided with an intermediate inlet which is connected to the leak detecting device input via a connecting conduit. In the course of the connecting conduit, a valve of adjustable conductance is provided. The adjustable valve serves as an adjustable throttle while the pressure at the leak detecting device input is so high that an unthrottled connecting conduit in the mass spectrometer would lead to an internal pressure that is so high that the mass spectrometer does not work. For reasons of costs, the inner diameter of the valve is limited to a diameter as small as possible. Thereby, however, a considerable throttle effect is maintained, reducing the suction capacity, even if the valve is completely open.